1. Field of the Invention
The present invention relates to radio navigation systems. More particularly, this invention pertains to apparatus for use in a radio navigation system of the type in which the phase difference between synchronized generated and received coded signals is utilized to determine range.
2. Description of the Prior Art
In the field of radio navigation, the current state of the art often favors the use of systems that utilize the phase comparison of coded signals to determine distance between a known point and a receiver. By making a number of distance determinations from the speed of light times the phase difference between a transmitted signal and a "synthetic" coded signal generated at the receiver, the two signals being synchronized in time, the requisite component distances can be obtained for subsequent determination of position using triangulation.
The accuracies of measurements obtained as above have often been impressive. This has led, in turn, to the employment of the above-described techniques in advanced satellite-based navigation systems such as the NAVSTAR Global Positioning System ("GPS"). In such a system, multiple satellites, the position of each of which is known, serve as points of origin of transmissions, each transmission comprising a uniquely-coded serial bit stream sent in repeating epochs. Distance determinations are made by observing the phase difference between the coded transmissions and the (identically-coded) synthetic signal generated at the receiver. Fixings of multiple satellites are made to survey the receiver's location in three-dimensional space. This requires the generation of multiple codes at the receiver and the sequential receipt and detection of the uniquely coded signal from multiple (usually four) satellite transmitters of known location. In the Navstar system, for instance, a potential of thirty-two (32) satellites of known location are available as points of reference for worldwide radio navigation.
The signals used in the Navstar system comprise pseudo-noise bit or "chip" sequences known as "C/A-codes". Each C/A-code consists of 1023 digital data bits arranged in a "Gold" code formed as the product of two 1023 bit pseudo-noise (PN) codes having a phase shift there between. Mathematically stated, EQU C/A(n)=G.sub.1 (n)+G.sub.2 (n+N)
where N determines the phase shift between the G.sub.1 and the G.sub.2 codes. The G.sub.1 and G.sub.2 codes are, in turn, defined by corresponding generator polynomials G.sub.1 (x) and G.sub.2 (x): EQU G.sub.1 (x)=1+x.sup.3 +x.sup.10 EQU G.sub.2 (x)=1+x.sup.2 +x.sup.3 +x.sup.6 +x.sup.8 +x.sup.9 +x.sup.10
A total of 1025 unique C/A-codes as above, exist, thirty-two (32) of which are utilized for the NAVSTAR signals. The phase shifts, N, defining the selected Navstar codes, are listed below:
______________________________________ Code (Satellite) Number Phase Shift (N) ______________________________________ 1 5 2 6 3 7 4 8 5 17 6 18 7 139 8 140 9 141 10 251 11 252 12 254 13 255 14 256 15 257 16 258 17 469 18 470 19 471 20 472 21 473 22 474 23 509 24 512 25 513 26 514 27 515 28 516 29 859 30 860 31 861 32 862 ______________________________________
The C/A-codes are transmitted at a rate of 1.023 MBPS (mega bits per second); thus each code epoch repeats every millisecond.
In accordance with the power of ten components of the G.sub.1 and G.sub.2 polynomials, present day systems for generating and tracking coded phantom signals at the receiver are generally implemented by means of two ten stage shift register arrangements. Each coded epoch transmitted requires regeneration of a unique coded sequence that is associated with a preselected reference satellite. Each ten stage arrangement of shift registers may, of course, only be dedicated to a single one of the thirty-two C/A codes and, thus, multiple receiver modules (usually four) must be employed.
The use of ten stage registers and multiple receiver arrangements is costly, degrades system reliability and may result in excessive power consumption. More significantly, such systems are relatively slow in switching from one transmission to another. This can result in degraded tracking performance since large shifts in code phase between transmitted and phantom coded signals result in significant dead time (non-tracking) intervals, effectively reducing signal-to-noise ratio (SNR).